Interconnects allow various parts of a semiconductor device to be electrically connected with other parts of the semiconductor device. Unfortunately, the processes that form interconnects typically introduce mobile ions that degrade device reliability. Mobile ions, such as sodium, lithium, potassium, calcium, and magnesium, typically come from two sources: during a metal etching step and from an organic solvent typically used in removing a photoresist masking layer. The conventional wisdom of those skilled in the art is that mobile ions introduced during the metal etching steps lie only on exposed surfaces of an insulating layer or an interconnect formed during the metal etching process. A quick deionized water rinse after photoresist removal should remove virtually all of the mobile ions if they lie on exposed surfaces. Semiconductor devices that only have a deionized water rinse after photoresist removal still have unacceptably high device reliability problems.
Organic masking layer solvents include mobile ions. As used in this specification, an organic masking layer solvent is a chemical that is capable of readily removing an organic masking layer (i.e., photoresist, etc.). Examples of organic masking layer solvents include ketones (2-propanone (acetone), etc.), aliphatic hydrocarbons (n-heptane, etc.), alkali-amines (tetramethyl ammonium hydroxide, etc.), and aryl hydrocarbons (toluene, phenol, etc.). Examples of chemicals that are not organic masking layer solvents include alcohols (methanol, ethanol, 2-propanol (isopropyl alcohol), or the like) and glycols (methanediol (methylene glycol), 1,2-ethanediol (ethylene glycol), 1,2-propanediol (propylene glycol), or the like). These latter chemicals typically have at least one hydroxyl group for no more than ten carbon atoms within the molecule, wherein that hydroxyl group is directly attached to a carbon atom other than a carbon atom that is part of an aryl radical (i.e., not phenol). Although the alcohols and/or glycols may attack an organic masking layer, the rate of removing the organic masking layer typically is slow enough that it does not make the alcohols and/or glycols a chemical that readily removes an organic masking layer.
Many photoresist removal processes after a metal etching step use an organic masking layer solvent by itself or an aggregation of plasma ashing and an organic masking layer solvent. Many commercially-available organic solvents have mobile ions concentrations that are measured in parts per million. High-purity organic solvents are available that have mobile ions concentrations as low as about 10 parts per billion. However, these high purity organic solutions may still add mobile ion contamination to semiconductor devices. The cost of the organic solvents increase dramatically with higher purity.
Resist-etch-back processing sequences may also introduce mobile ions into a semiconductor device typically during a plasma etching step. Once again, mobile ions are undesired, and their concentration level in semiconductor devices should be kept as low as possible.